fix: more accurate default value handling

[dummdidumm]

• don't call fallback values eagerly, only when it's actually needed. Avoids potential unwanted side effects
• use derived_safe_equals to memoize results of destructured snippet/each context values with default values to ensure they're only recalculated when their dependencies change. Avoids unstable default values getting called multiple times yielding different results. Use derived_safe_equals to ensure new values are always set, even when mutated. fixes Svelte 5: A snippet default value is inconsistent #11143

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[changeset-bot]

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Latest commit: 7511b16

The changes in this PR will be included in the next version bump.

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